"When people would ask us who a 'dream guest star' for the show would be, we would always joke and say Stephen Hawking," the show's executive producer, Bill Prady, said in a statement when Hawking's appearance was announced in March. "In fact, we're not exactly sure how we got him. It's the kind of mystery that could only be understood by, say, a Stephen Hawking."

Hawking, who has amyotrophic lateral sclerosis (ALS), has put a face on modern physics. He says he is fascinated by variations in the cosmic microwave background, the telltale evidence left from the big bang. And he hopes the Large Hadron Collider - the giant supercollider in the Alps - will help find evidence to support the so-called theory of everything, which would unite gravity, magnetism and the other fundamental forces in nature.

He has also expressed worry that human beings may be undone by their own technology - that they'll let nuclear weapons or climate change get out of control before they know fully how to keep them under control. In a new documentary, " Surviving Progress," which opens today in New York, he says, "We're entering an increasingly dangerous period of our history. Our genetic code still carries selfish and aggressive instincts that were of survival advantage in the past. But I'm an optimist."

In January, when he turned 70, the magazine New Scientist asked him, "What do you think most about during the day?"